Seismic surveying



March 28, 1939. B. B. WEATHERBY SEISMIC SURVEYING Filed April l5. 1937 2 Sheets-Sheet l INVENTOR @n rf ATTORNEYS han B. B. WEATHIERBY 2 Sheets-Sheet 2 o No No ANo SEISMIC SURVEYING Filed April l5, 1957 March 28, 1939.

I. #INVENTOR BY d 'm I ATToRNEY.

Patented Mar. 28, 1939 UNlTED ST'ES PATENT oFFlc seismic soavamc Application April 15, 1937, Serial No. 13:1,013

2 Claims.

This invention relates to sub-surface seismic surveying and is particularly applicable in the art of making reflection seismic surveys by the correlation method. 'I'he basis of this method is the 5 similarity in appearance of records of reections obtained from a. given sub-surface stratographic horizon extending over a considerable area. To obtain such reection records, explosive charges are detonated in a plurality of bore holes distributed over the area and reflection waves are received at recording stations associated with the bore holes, there being a receiving station individual to each bore hole. Because the abovereferred to similarity of records, it is possible to l5 calculate the variation in depth of the reecting horizon over considerable areas by first correlating the reection records at different points in the area under investigation, that is, by identifying corresponding phases of the same reflection on each record and then calculating'the depth to the reflecting horizon at each point. These calculations are based upon the time intervals between the detonation of the seismic charge and the arrival at several spaced receiving stations-of the same phase of the corresponding reflections as indicated by the several records. Obviously, this method cannot be used if the appearance or character of the record of the reflection coming from the sub-surface horizon being mapped changes radically from one recording position to another.

to identify corresponding phases of the reilections on the several records, then the variation in the depth to the reecting horizon cannot be accurately followed.

I h ave found that a serious cause of character variation in reections coming from a 'given subsurface horizon is directly connected with the 40 nature of the impulse generated at the shot position by the detonation of the seismic charge. I have also found that in an area characterized by a surface layer comprising different types of earth strata, such character variations may be produced merely by detonating seismic charges in the different strata and may be of such degree that a reflection record produced by a charge detonated in one stratum will produce a record which is uncorrelatable with other records produced by the detonation of explosive charges in the same area. Considerable difficulty, therefore, may be experienced in surveying such area by the correlation method.

An object of this invention is to minimize the Possibility of miscorrelating reflection records.

If the changes are so great that it' becomes impossible to correlate the records, i. e.k

(Cl. ISI-0.5)

According to this invention, an explosive charge is provided which extends into more than one stratum as distinguished from the concentrated charge heretofore used. The eect of the explosive charge is thereby exerted in more than 5 one stratum instead of being confined to a single stratum. A variation in the character of the resulting wave due to the nature of one stratum will, therefore, have but little effect upon the resulting total impulse. This procedure, therel0 fore, minimizes the character changes caused by variations in the form of the exciting impulse.

\Other objects, novel features and advantages of this invention will become apparent from the following speciflcation and accompanying drawings, 15 in which f Fig. 1 diagrammatically illustrates one embodiment of the inventiom..

Fig. 2 diagrammatically illustrates another embodiment of the invention, and 20 Fig. 3 diagrammatically illustrates the area under survey.

In Fig. 1, A represents a bore hole in the earth of any desired depth while S1S2, S3, S4, Ss and Se. represent seismometers arranged at different dis- 25 tances from the mouth of the hole.. The seismometers are suitably connected to av recorder R, which is designed to produce records of the Waves received by the various seismometers. In the bore hole is provided an explosive charge O diS- 30 tributed in a continuous line from the bottom' of the hole upward and traversing one or different types of earth strata as indicated in the drawings. A single blasting cap, located at the top of the charge, is suiiicient to detonate the charge sub- 35 stantially instantaneously since the velocity of detonation of the blasting gelatin commonly used is of the order of 13,000 feet per second. An even closer approximation to a simultaneous detonation of the entire charge may be obtained by the 40 use of a primer commonly known by the name Cordeau. The lines P1, P2, Ps, P4, P5 and Pa indicate the paths followed by waves produced by the detonation of the charge O and reflected from the sub-surface horizon H tothe various seis- 45 mometers. The hole A may be of any depth but for illustration purposes, it may be considered as 100 feet deep with the charge O extending froma point near the bottom of the hole to a point near the top of the hole.

In Fig. 2, A represents a hole bored in the earth to any desired depth while S1, S2, S3, S4, S5 and Se represent seismometers arranged at different distances from the mouth of the hole. The seismometers are suitably connected to a recorder R 65 ydepths inl closely adjacent holes. yonly that the different depth charges be detonated in holesr substantially the'same distance from ther seismometers and in the same direction.

'which is' designed to produceA traces of the waves received by the various seismometers. The rs!-i erence characters O1. Oa, Ox and 04 indicate explosive 'charles located at different depths in the hole. yThese charges are connected through a conductor C with a detonator B of the usual tm for detonatins an' explosive charge. The lines P1. Pz. Ps. Pi. P and Pa indicate rthe paths followed by waves produced by the detonation of the charge O1 andreiiected lfrom lthe sub-surface horizon H to the various seismometers. sake of clearness, the wave paths are shown only in connection with the charte O1. The hole A may be of any depth. but for illustration purposes,

the hole may be' considered as 100 feetdeep with they chargeOr arranged at the bottom of the hole,

thecharge Oz arranged at a depth of 80 feet.the charge O: arranged at a depth of 70 feet and the charge O4 arrangedat the depth of 45 feet. The f charge O1 need not be arranged at :thebottom of the hole, but the hole may be continued some distance therebeyond.

In practicing this invention, ity is-immaterial whether 'the records be taken with the seismic charges planted'y at Adifferent depths in the same hole or whether' they are planted at different It is essential Althoughl the above-described method is of primary importance in 'seismlcfsurveying by the correlation method, it is also applicable in the f method cfseismic surveying commonly known as lfrom one recording position' to another.

dip shooting. In the latter method, .no at tempt is made to correlate the reflections obtained yIt is merely necessary to' piel:y ,out corresponding phases of the reflections appearing on each indif vldual'reccrd, Asince the reflection 'recordsA are used merely ln'cal'culating thedip of the subsurface bed. Although variations in character from one recording position to another are, therefore, of little importance, it has been found that vin some areas more distinct reections are obtained when the depth at which the seismic charge is planted is varied. In this type of surveying, therefore, the procedure above described is also of considerable value.

arsnsvs In the event thatthe explosive charge is detonated in a continuous line from the bottom ci fifteen lectfromA the surface in order toavold blowing out. In the event that separate chin-liesv are used, the exact' depths at which the charges i are rplanted are of little importance and also the distances between the chargesv beary no speelde relationship to eachother. In this case also it is desirabley to .plant lthe uppermost charac ynot less than 15 ft. below the surface. f f f In Fig. 3 is illustrated a common arrangement the hole upward. it is desirable that; the upper v .end of they charge be keptbslow approximately of shot pointsin anA area beine surveyed by the correlation method. In this lliure', A represent! the shot hole illustrated in Fiss. 1 and 2, while R rrepresents the recording position also illustratedy f in Figs. 1 and 2', each recording position being equipped lwith a plurality of selsmometers as f shown in said two. nsures. Other boreholes in the area are indicated by the reference characters A1., A: and A: while the recording stations associated with such bore holes are indicated by the reference characters R1, Rn and Ra. seismometers may be placed at any suitable distance from the mouth of the borehole but for illustration purposes theyvmay be assumed. to be 1 spaced respectively 2000 feet, 2200 feet, 2400 feet, l2600 feet, 2800 feet and 3000 feet from the mouth of the bore hole.

I claim: e 1. In the method of seismic surveying by correlation of reflection recordaproduced yby ldetonation of explosive charges in bore holes in an area characterized lby a surface llayer lcomposed of 2. The method according to claim 1 whereinv said explosive charge is continuous and extends from substantially the bottom of said bore hole substantially to the top thereof.

BENJAMIN B. WEATHERBY.

receiving reflected- 

